Organic semiconductor devices are attracting attention as a next-generation semiconductor device because thin film fabrication can be conducted at low costs, unique characteristics such as mechanical flexibility are retained, and materials themselves of the organic semiconductor devices can easily be modified by an organic synthetic technology. Display devices with use of organic light emitting diodes (OLEDs) have already been put in practical use, and organic field effect transistors (OFETs) as a drive device for flexible display devices and/or organic photovoltaic cells (OPVs) are being studied toward practical application.
In such organic semiconductors, a hole having positive charge and an electron having negative charge render semiconductor their characteristics. It is possible, until now, to accurately determine the valence states, which relate to the hole conduction, by photoemission spectroscopy and/or photoemission yield spectroscopy. On the contrary, since there is no method for accurately determine unoccupied states, that are electron conduction levels, the behavior of electrons in organic semiconductors is not yet fully known. Under these circumstances, a device for accurately measuring the unoccupied states of the organic semiconductors is needed.
To easily measure the unoccupied states of the organic semiconductors, methods as shown below are used: (1) a method for estimating an electron affinity, which is the lower end energy of the unoccupied-states, based on a reduction potential obtained by an electrochemical approach (cyclic voltammetry) in solution; and (2) a method for estimating the lower end energy of the unoccupied-states by adding a band gap calculated from an optical absorption spectrum to an ionization energy (which corresponds to the upper end energy of the HOMO level) determined by photoemission spectroscopy.
However, in the case of the electrochemical method (1), the reduction potential of molecules in solution is often largely different from the electron affinity in solid, which makes measured values inaccurate. In the case of the method (2), the band gap obtained from the optical absorption spectrum is often smaller than an actual gap due to the influence of excitons. In systems having a large electron correlation such as molecular systems, injected and released charges and remaining electrons have a large correlation. As a consequence, the method (2) has a problem that correct unoccupied-states energy cannot be obtained.
As another method for measuring the unoccupied states, inverse photoemission spectroscopy which is a time inversion process of photoemission spectroscopy is also known. In the inverse photoemission spectroscopy, a sample is irradiated with an electron beam with uniform energy, and light emitted thereby is detected, so that density of states of the unoccupied-states can be examined. Based on this spectrum, in particular, electron affinity of solids, which is an index of the unoccupied states, can be obtained. In principle, this method is considered to provide most reliable values (Non Patent Literature 1 to Non Patent Literature 4).